The protein tubulin plays a vital role in the life of all eukaryotic cells. Microtubules, made most of tubulin, are involved in organelle movement, separation of chromosomes during cell division, and maintenance of cell shape, for example. The assembly and disassembly of microtubules at certain times are critical steps in the cell cycle. It is important to understand how tubulin molecules interact with each other as well as with a large number of other proteins, in order to have a full understanding of the life of the cell and it will be necessary to know the structure of tubulin before we can understand these interactions. We are studying tubulin structure by electron crystallography of two-dimensional, crystalline sheets that form in the presence of zinc ions. These sheets are in ideal form for study in the electron microscope, and circumvent the problem that attempts have so far been unsuccessful to obtain crystals of tubulin for x-ray crystallography. Electron crystallographic methods have been developed in recent years to the point where it is possible to derive a three-dimensional density map at a resolution sufficient to build an atomic model of the structure. We have now achieved our first goal of constructing the initial tubulin model, which gives great insight into tubulin's varied functions. We will extend the model to get a better picture of how the tubulin molecule fits into a microtubule. Combined with atomic structures of motor molecules or other microtubule-associated proteins, we should also be able to understand in detail, for example, how the proteins interact to produce directed motion and to enhance microtubule stability. Even at lower resolution we are able to gain substantial understanding of the relation between tubulin's structure and function and interactions with other molecules. In addition, a number of clinically useful drugs interact with tubulin, and the structure of tubulin-drug complexes will provides a basis for understanding these interactions and possible drug analogs that may modulate the interactions and provide more specific drug effects.